For many coating applications such as automotive coatings, aerospace coatings, industrial coatings and architectural coatings, dark colors, such as black and dark blue are particularly desirable for aesthetic purposes. However, dark colored coatings have historically been susceptible to absorption of near-infrared radiation because they often rely on the use of pigments, such as carbon black, that absorb near-infrared radiation in addition to visible radiation. Near-infrared radiation, i.e., light energy having a wavelength of from 700 to 2500 nanometers, constitutes over 50% of the solar energy that reaches the earth's surface. Heat is a direct consequence of the absorption of near-infrared radiation. As a result, dark colored coatings have historically been susceptible to substantially increased temperatures, particularly on sunny days, which is often undesirable for many reasons. Thus, solar heat (near-infrared) reflecting coatings have been desired.
In Principles and formulations for organic coatings with tailored infrared properties, Progress in Organic Coatings, 20:1-25 (1992) (“Brady”) formulation approaches for achieving solar heat reflecting coatings are described. In one approach, a two layer coating system is employed in which an upper layer is colored with pigments that absorb visible radiation but are transparent to near-infrared radiation, such as organic black pigments (perylene blacks are mentioned) or other organic pigments (phthalocyanine blues and greens and carbazole dioxazine violet are identified), and an underlayer, such as a highly reflective white undercoat, that reflects near-infrared radiation, reduces the temperature increase of the coating system. An example of such a coating system is also described in United States Patent Application Publication No. 2004/0191540 A1.
There are certain drawbacks associated with prior versions of such coating systems. One drawback has been difficulty in achieving coatings that exhibit a jet black color. This is because the infrared transparent organic pigments used in the upper layer have a tendency to scatter light compared to the infrared-absorbing carbon black pigment. This problem is exacerbated in thin film areas and coating edges, where the coating may not appear black at all. Another drawback has been the inability to achieve such coating systems that exhibit color “stability” in which the color of the coating does not change appreciably after exposure to weathering conditions.
As a result, it would desirable to provide coating compositions that can produce coatings that are transparent to infrared radiation and can exhibit a “stable” color, such as a jet black, even in thin film areas and coating edges. It would also be desirable to provide coating systems that include coatings deposited from such compositions.